1. Field of the Invention
The present invention is in the field of optical fiber systems, and, more particularly, is in the field of optical fiber lasers and optical fiber amplifiers.
2. Description of the Related Art
Silica-based fibers are excellent media for high-power lasers for two basic reasons. Silica-based fibers exhibit an extremely high optical damage threshold. Silica-based fibers have a high surface-to-volume ratio, which makes heat dissipation very effective. However, in a conventional fiber laser pumped with a laser diode, the pump light is coupled into the fiber core, which imposes a major limitation on the maximum pump power that can be launched into the fiber. The limitation arises because efficient coupling of the laser diode output into the single-mode fiber core requires the use of a single-mode laser diode. The output power of such pump sources is currently limited to a fraction of a watt by the damage threshold of the semiconductor material in the laser diode. The limit of the pump output power significantly limits the maximum output power of the fiber laser.
The maximum output power of the fiber laser is also limited by the efficiency with which the beam of a laser diode can be coupled into a fiber core. This efficiency is typically about 60% or less because of astigmatism of the laser diode beam. Thus, only a fraction of the available pump power is usable. Since the power conversion of the fiber laser is lower than unity, the brightness of such a fiber laser is typically lower than that of the pump source.
A solution to this problem is to use a double-clad fiber as described, for example, in E. Snitzer et al., Double-clad, offset core Nd fiber laser, Proceedings of Conference on Optical Fiber Sensors, Postdeadline paper PD5, 1988. A double-clad fiber comprises a single-mode core doped with a laser ion. The core is surrounded by a first (or inner) cladding of lower refractive index. The inner cladding is surrounded by a second (or outer) cladding of still lower index. The inner cladding thus forms a second, highly multimoded waveguide. The two claddings generally do not contain laser ions.
A double-clad fiber is advantageous because a large amount of pump power (up to 100 s of watts) from low-spatial-coherence, high-power laser diode pumps can be efficiently coupled into the inner cladding because the inner cladding has a large area and a high numerical aperture (NA) approximately matched to the emitting area and the numerical aperture of the pump source (or of the multimode fiber that delivers the pump power, in the case of a fiber-pigtailed laser diode). Another advantage of the double-clad fiber is that the alignment tolerance in coupling to the large-area inner cladding is typically tens of microns rather than the sub-micron tolerance for coupling into a single-mode fiber core. See, for example, Th. Weber et al., Cladding-pumped fiber laser, IEEE Journal of Quantum Electronics, Vol. 31, No. 2, February 1995, pages 326–329.
Double-clad fibers have been used since their inception to produce lasers with several rare-earth ions, including, for example, Nd3+, Yb3+, and Tm3+ included in the core. Reports indicate rapidly increasing fiber laser output powers, from the original tens of milliwatts to a recently reported 110 watts in a Yb-doped fiber laser. See, for example, V. Dominic et al., 110 W fibre laser, Electronics Letters, Vol. 35, No. 14, July 1999, pages 1158–1160. Double-clad fiber lasers with output powers in the 10-watt to 20-watt range have already reached the commercial market and are finding practical applications.
Increasing the pump absorption in a double-clad fiber generally requires an increase in fiber length which may result in increased loss at the laser and pump wavelengths and reduced fiber laser output. Since the spatial overlap between the pump cladding modes and the dopant distribution in the core is relatively small, the pump power absorbed per unit length is reduced compared to a core-pumped fiber, and a longer fiber is required to excite the same total number of dopant ions. Typically, the length increase is ten-fold or more. In a four-level transition (where ground-state absorption (GSA) is non-existent), this length increase results in a negligible increase in the laser absorption loss and therefore a negligible change in the threshold. These points are illustrated by an experimental study of the dependence of the threshold and slope efficiency on fiber length in a double-clad Nd-doped fiber laser, as described, for example, in Th. Weber et al., Cladding-pumped fiber laser (cited above). However, GSA is substantial in a three-level transition. Thus, the fiber length increase results in a large increase in signal absorption loss and threshold. Double-clad fibers are in general not as effective for three-level transitions.
A large body of literature now exists on the theoretical investigations and design of double-clad fiber profiles to optimize power transfer per unit length from the pump cladding modes to the dopant. See, for example, L. Zenteno, High-power double-clad fiber lasers, Journal of Lightwave Technology, Vol. 11, No. 9, September 1993, pages 1435–1446; and V. Reichel et al., High-power single-mode Nd-doped fiber-laser, in Solid State Lasers VII, Proceedings of SPIE, Vol. 3265, 1998, pages 192–199. One difficulty encountered with respect to double-clad fiber is that only the HE1m family of modes have intensity at the center of a fiber when the inner cladding has a circular cross section. Since most of the pump is launched as helical rays, most of the pump power misses the core. See, for example, E. Snitzer et al., Observed dielectric waveguide modes in the visible spectrum, Journal of Optical Society of America, Vol. 51, No. 5, May 1961, pages 499–505.
One solution for increasing the pump power transfer efficiency offsets the core from the center of the inner cladding, as described, for example, in E. Snitzer et al., Double-clad, offset core Nd fiber laser (cited above) and in H. Po et al., Double-clad high brightness Nd fiber laser pumped by GaAlAs phased array, Proceedings of Optical Fiber Communication '89, Postdeadline paper PD7, 1989. In one measurement involving an Nd-doped fiber, the pump absorption when the core was centered on the cladding was only 5% of what it would be if the same amount of dopant was uniformly distributed across the inner cladding. By offsetting the core to near the edge of the inner cladding, the pump absorption increased to 28%, as described, for example, in H. Po et al., Double-clad high brightness Nd fiber laser pumped by GaAlAs phased array (cited above).
Another solution for increasing the pump power transfer efficiency alters the shape of the inner cladding to support modes that fill the multimode cladding. For example, rectangular claddings, D-shaped claddings, and scalloped claddings have been proposed and evaluated in A. Liu et al., The absorption characteristics of circular, offset, and rectangular double-clad fibers, Optics Communications, Vol. 132, No. 5–6, December 1996, pages 511–518, in V. Reichel et al., High-power single-mode Nd-doped fiber-laser, (cited above), and in D. J. DiGiovanni et al., High-power fiber lasers, Optics and Photonics News, Vol. 10, No. 1, January 1999, pages 26–30.
In a D-shaped cladding, the helical rays are coupled to the meridional rays and all pump rays pass through the core, as illustrated, for example, in V. Reichel et al., High-power single-mode Nd-doped fiber-laser (cited above). It was pointed out early that a rectangular shape is particularly attractive because it matches the output pattern of a multiple-stripe laser diodes, thereby minimizing the area of the inner cladding and thus minimizing the required fiber length. See, for example, H. Po et al., Double-clad high brightness Nd fiber laser pumped by GaAlAs phased array (cited above). Subsequent simulations showed that both a rectangular and a D-shaped cladding can provide a pump absorption in excess of 90%, or over one order of magnitude larger than a circular cladding. See, for example, V. Reichel et al., High-power single-mode Nd-doped fiber-laser (cited above).
Helical pump rays have also been reduced by inducing mode mixing via bending the fiber. See, for example, A. Liu et al., The absorption characteristics of circular, offset, and rectangular double-clad fibers (cited above); H. Zellmer et al., Double-clad fiber laser with 30 W output power, Proceedings of Optical Amplifiers and their Applications, OSA Trends in Optics and Photonics Series, Vol. 16, July 1997, pages 137–140; H. Zellmer et al., High-power cw neodymium-doped fiber laser operating at 9.2 W with high beam quality, Optics Letters, Vol. 20, No. 6, March 1995, pages 578–580; and A. Liu et al., The absorption characteristics of rare earth doped circular double-clad fibers, Optical Review, Vol. 3, No. 4, July–August 1996, pages 276–281. Helical pump rays have also been reduced by mode scrambling, as described, for example, in A. Tünnermann, High-power Nd double-clad fiber lasers, Proceedings of Conference on Lasers and Electro-Optics, CLEO'96, Paper CFJ1, 1996, page 528. A particularly effective design involves bending the fiber in a kidney-shaped geometry, as described, for example, in H. Zellmer et al., Double-clad fiber laser with 30 W output power (cited above). Theoretical studies of the absorption efficiency of various double-clad fiber profiles as a function of fiber length and bend radius can be found in the above-cited A. Liu et al., The absorption characteristics of circular, offset, and rectangular double-clad fibers, and in the above-cited H. Zellmer et al., Double-clad fiber laser with 30 W output power. 
Alternative techniques can be used to couple pump light into a double-clad fiber. For example, side-pumping via a prism is illustrated in Th. Weber et al., A longitudinal and side-pumped single transverse mode double-clad fiber laser with a special silicone coating, Optics Communication, Vol. 115, No. 1–2, March 1995, pages 99–104; and in Th. Weber et al., Side-pumped fiber laser, Applied Physics B, Vol. 63, No. 2, August 1996, pages 131–134. If the outer cladding is made of a polymer such as silicone, it can be locally stripped. A prism of appropriate angle is clamped against the exposed inner cladding and pump light is coupled through the prism into the cladding. This method has proved to transfer the pump power to the core quite effectively, as described, for example, in Th. Weber et al., A longitudinal and side-pumped single transverse mode double-clad fiber laser with a special silicone coating, cited above.) This method has also been applied to launch pump light at two different locations simultaneously into the same fiber in, for example, Th. Weber et al., Side-pumped fiber laser (cited above).
Although such other techniques are available, experimental measurements in a Ho/Tm-doped fiber laser have shown that core pumping is still the most efficient method. See, for example, C. Ghisler et al., Cladding-pumping of a Tm3+:Ho3+ silica fibre laser, Optics Communication, Vol. 132, No. 5–6, December 1996, pages 474–478. In these experiments, cladding pumping produced a higher threshold (by a factor of approximately 2.3) and a lower slope efficiency (by approximately 43%) than core pumping. Side pumping yielded the same threshold as cladding pumping but a still lower slope efficiency (by approximately 18%).
Another method developed specifically to couple light from a high-power laser diode bar is described, for example, in L. A. Zenteno, Design of a device for pumping a double-clad fiber laser with a laser-diode bar, Applied Optics, Vol. 33, No. 31, November 1994, pages 7282–7287; and in H. Po et al., High power neodymium-doped single transverse mode fibre laser, Electronics Letters, Vol. 29, No. 17, August 1993, pages 1500–1501. This method uses a bundle comprising a large number of multimode fibers arranged at one end into an array shaped to match the laser-diode emitting area. For example, the bundle is arranged into a linear shape to match a long and thin laser diode bar. The pump light is coupled into the bundle with a cylindrical lens. At the output end of the bundle, the fibers are arranged into a two-dimensional cross-section matched to the double-clad fiber pump cladding.
Another method demonstrated with a 1.5-μm Er/Yb fiber amplifier is described, for example, in L. Goldberg et al., V-groove side-pumped 1.5 μm fibre amplifier, Electronics Letters, Vol. 33, No. 25, December 1997, pages 2127–2129. In accordance with this method, a V groove is polished into the side of the pump cladding. Pump light is focused through the side of the fiber onto a polished face of the groove and reflected by total internal reflection into the pump cladding. When a V-groove coupler is used, as much as 76% of the output power of a laser diode array was coupled into a pump cladding. (See, for example, L. Goldberg et al., V-groove side-pumped 1.5 μm fibre amplifier (cited above).
Side pumping using a multimode fiber coupler has also been reported in, for example, V. Gapontsev et al., 25 kW peak power, wide-tunable-repetition-rate and pulse duration eye safe MOPFA laser, Proceedings of Conference on Lasers and Electro-Optics, CLEO'96, Vol. 9, Paper CTuU3, 1996, pages 209–210.